Rebecca Karberg of the General Services Administration inspects rebar at the site.

Photo: Lea Suzuki, The Chronicle

Rebecca Karberg of the General Services Administration inspects...

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Materials believed to the foundation of the original San Francisco City Hall that collapsed during the earthquake and fire of 1906 were discovered during landscaping work at a construction site at 50 U.N. Plaza on Friday, September 21, 2012 in San Francisco, Calif.

Crews working on a building project in San Francisco's Civic Center have unearthed the massive foundations of the old City Hall, a ghostly reminder of San Francisco's greatest disaster.

The imposing old City Hall collapsed in a shower of bricks, stone and steel in the 1906 earthquake. It was the largest municipal building west of Chicago and was so elaborate it took 25 years to build. The City Hall was supposed to be earthquake proof, but it collapsed in seconds after the great quake struck. It had been open for less than 10 years.

Its ruins were demolished in 1909, but workers digging under the sidewalk on Hyde Street near Fulton Street for a landscaping project struck something big Sept. 14 - bricks and concrete and steel reinforcing bars. They called archaeologists from the federal General Services Administration, which owns the adjacent former federal building at 50 United Nations Plaza.

They looked at old maps and old reports: It was the 1906 City Hall, all right. "We were surprised to see it," said Rebecca Karberg, historic preservation specialist for the GSA. "You really never know what's under the surface."

The wreckage of the old City Hall - a grandiose dome 300 feet high held up by the skeletal remains of a building - became a famous symbol of the '06 quake. The wrecked building was widely photographed, and the pictures were sold as postcards.

The cornerstone of City Hall was laid on Washington's Birthday, 1872, though site excavation started the year before. It was built on the site of the old Yerba Buena Cemetery, where perhaps as many as 9,000 San Franciscans were buried between 1850 and 1860. "The original 49ers," Karberg called them.

Shifting design

The site, just off Market Street where the Main Library now stands, was sandy and wet in the winter. There was an underground spring. The original design called for a building in the shape of the letter W, with columns and ornamental towers in the French Second Empire style.

Over the years, however, the design was changed more than once. There were big cost overruns and various scandals. It took so long to build, it became a municipal joke: "The new City Hall ruin," it was called.

Only a year after its 1897 opening, it was damaged in a minor earthquake, an ominous sign. "It was the proverbial disaster waiting to happen," wrote Stephen Tobriner in "Bracing for Disaster," a book about engineering in earthquake country.

Before dawn on April 18, 1906, the Big One struck.

Officer E.J. Plume was in the police station in the City Hall basement. He heard the pillars that held up the cornices and the cupola "go cracking with reports like cannon, then falling like thunder." The building "seemed to be reeling like the cabin of a ship in a storm."

The officers ran out, but City Hall was otherwise unoccupied. Had it been full of city workers, the death toll would have been huge.

After the quake, rumors circulated that contractors who built City Hall had cut corners: The great exterior columns, it was said, were hollow, filled with street sweepings.

But a report by architects after the disaster found the construction to have been solid; it was the design that failed. City Hall, they said, had been built "without any of the principles of the steel frame construction having been used."

So it was torn down and nearly forgotten. Portions of the cemetery - including graves of early pioneers - were found when the Federal Building at what is now U.N. Plaza was built in 1933; and again the 1990s and 2001, when both a corner of the old City Hall foundation and part of the cemetery were unearthed during construction of the Asian Art Museum and the library.

Big surprise

But this month's discovery was still a surprise. The General Services Administration is rebuilding the 1933 Federal Building and had evidently not expected to find anything of the area's past. The new findings provide a window into the past and, perhaps, an opportunity to learn something more about construction in earthquake territory.

"We enjoy history," said Joanne Grant, an archaeologist. "I'm not from San Francisco, so I have a lot to learn about the history here. But now, we are digging it up."

When the architects and historians are finished documenting the ruins of the old City Hall, and the landscaping project is ready to go, the crews will go back to work.